Among conventionally known photodevices, there are, as solid state photoelements, so-called solar batteries each of which can be obtained by forming a p-n junction near the surface of a crystal principally made of a silicon semiconductor. However, semiconductive crystals of high purity are indispensable as starting materials for such solar batteries. Corollary to this, the manufacturing cost of a solar battery is high and its use is thus limited to special applications only. Accordingly, such solar batteries are not considered to be as commonly usable energy conversion elements.
With a view toward solving the above-described drawback of conventional solar batteries, various studies have recently been carried out on photoelectrochemical cells which make use of inexpensive semiconductors such as sintered semiconductors and still function sufficiently.
However, such photoelectrochemical cells are accompanied by the problem that their anodes would be dissolved due to the photoelectrode reaction regardless of the type of semiconductive material to be used, thereby resulting in a shortened lifetime of the anodes. Of course, there are some exceptional semiconductive electrodes which do not practically involve such a dissolution problem, e.g., tin oxide, titanium dioxide, strontium titanate, etc. However, all of these semiconductors exhibit sensitivity only to radiation in the ultraviolet region. Thus, they are unsuitable as conversion elements for solar energy whose spectrum is distributed widely in the visible and near-infrared radiation regions and have thus not been adapted for the conversion of solar energy to electric energy.
In view of the above-described drawbacks of the prior art electrodes, many attempts have been made to expand to a longer wavelength side the sensible wavelength region of a stable semiconductor which is sensitive to ultraviolet light but does not develop the dissolution problem due to the photoelectrode reaction. Such attempts were all dependent on dye sensitization, in other words, it was tried in each of the attempts to convert photoenergy of longer wavelengths, which cannot be absorbed by a semiconductor electrode per se, to electric energy by adding to an electrolyte solution a dye which is capable of absorbing radiation of longer wavelengths than the characteristic absorption wavelengths of the semiconductor electrode.
It has been made clear through a measurement of a photo electromotive force using monochromatic light that, in a photoelectrochemical cell whose electrolyte solution contains a sensitizer dye, photoenergy having a wavelength longer than the characteristic absorption wavelength of its semiconductor per se can be converted in part to electric energy. However, its energy conversion efficiency is extremely low and, consequently, it has been concluded that such photoelectrochemical cells are not suited for practical applications.
By the way, as a cause of the low conversion efficiency described above, it has been known that the dye dissolved in an electrolyte solution does not take any part in the dye sensitization effect but only the dye which is absorbed on the electrode surface contributes to the dye sensitization effect. However, no means have been proposed to solve the above-described principal problem of a low conversion efficiency.
In view of the above drawbacks of the prior art photoelectrochemical cells, an object of this invention is to provide a chemically-modified photoelectrochemical cell which can convert photoenergy of a longer wavelength to electric energy with a high efficiency by making the use of the dye sensitization effect, thereby adding sufficient practical usability to the photodevices.